1. Field of the Invention
The present invention relates generally to automatic transmissions and more specifically to a control arrangement therefore which improves upshifting particularly during "power off" modes of engine operation when the engine is not induced to produce torque.
2. Description of the Prior Art
When an engine switches to a power off mode of operation, as shown in the shift pattern illustrated in FIG. 8 (note that only the upshift lines are shown), in response to the engine throttle valve closing from a level equal to or higher than point X, to a level denoted by Y, with the vehicle running at a given vehicle speed, the 1-2 shift line is crossed and the transmission is conditioned to undergo a corresponding upshift.
In response to the closure of the throttle valve, the idle switch is closed and for a predetermined period of time following this event, depending on the gear ratio developed by the gears which are operatively engaged, the rotational speed of the input and output shafts do not synchronize. This asynchronization results in a backlash being produced in the differential gear. This of course causes both noise and a jerking of the vehicle.
In order to overcome this drawback a less preferred transmission control embodiment (disclosed in Japanese Patent Application No. 61-282252 filed by the same entity as the instant application is assigned) has been proposed.
With this arrangement, as indicated in FIG. 9, when the driver releases the accelerator pedal in a manner which permits the throttle to close from a fully open or near fully open position, to a fully closed one, at the time point t1 a decision to implement a 1-2 upshift is produced is response to the throttle valve assuming a fully closed position and triggering an idle switch which is associated therewith.
Following the issuance of this signal, the engine speed NE and the rotational speed of the torque converter output element NT reduce. For the sake of explanation let it be assumed that the gear ratio which will be produced by the second speed gearing is 1.619, whereby the rotational speed NO of the transmission output shaft will become NT.times.1.619.
However, at this time the transmission is still conditioned to produce first gear ratio the current rotational speed ratio G=NT/NO As shown, this value decreases.
When G=g1 (g1=a predetermined value) such as indicated at time t2 in this figure, a shift solenoid A is de-energized (conditioned to assume an OFF state) and the second speed engagement pressure permitted to build-up in the appropriate friction elements and condition the transmission to produce second speed.
This control induces the situation wherein the input and output rotational speeds NT, NO become essentially equal during the initial stages of the second speed engagement and the illustrated smooth transmission output torque characteristics are derived. These characteristics of course are such that the above mentioned backlash and associated problems do not occur.
However, this arrangement has suffered from the drawback that the above timing control is suited to controlling a power off type 1-2 upshift only in a narrow vehicle speed range, and as the vehicle speed deviates from this range, the above described input and output shaft rotational speed synchronization cannot be achieved at the desired timing with respect to the second speed engagement operation and considerable shift shock tends to be generated under such conditions.